


Poisons 101

by Neon_Sleep



Series: Insiders [2]
Category: Fire Emblem Series, Fire Emblem: Fuukasetsugetsu | Fire Emblem: Three Houses
Genre: Developing Friendships, Developing Relationship, F/M, In which I make up all the science, Student!Byleth, Wild Card Byleth
Language: English
Status: Completed
Published: 2019-10-30
Updated: 2019-10-30
Packaged: 2021-01-12 23:49:35
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,862
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21234626
Author URL: https://archiveofourown.org/users/Neon_Sleep/pseuds/Neon_Sleep
Summary: Byleth decides she'd like to learn poisons in exchange for extra sword mentoring and tolerance of his endless questions. Claude finds out it isn't easy being a Professor.





	Poisons 101

**Author's Note:**

> This story takes place before "In the Library", but both are standalone one-shots. I live for curious Byleth.

“Two ounces of water and five petals of Ailell lavender, brought up to 225 degrees exactly. Alright By, will u hand me the vial of Morfis amanita clippings?” It was a quiet Saturday afternoon at the monastery and despite not having classes, Claude had taken on the role of “Teach” for the day.

Byleth hummed in affirmation, gently flipping through the dozens of vials Claude had in his case. The glasses tinked together like tiny bells and her fascination grew with each one she picked up.

“What’s this one?” she asked holding up a vial containing a glowing bright green gel.

“It’s a tree sap that when mixed with water will emit its own light. The mushrooms please?”

“Hmm.” She looked at the glowing vial with her typical expressionless face before gently placing it back and continuing her search. Claude watched her with amusement. Though this demonstration was time-sensitive, he was content to witness her wonder at all the herbs and ingredients he kept in his collection. Her face was still a closed book to him, but he recognized from her body language and subtle vocal cues the same curiosity that had possessed him to learn the delicate science of poisons and potions.

“What about this one?” Byleth held up a vial with a shining silver liquid sloshing inside.

“No, dOn’T touCH THAT!!” Claude reached across the table and gently snatched the vial out of her hand, its contents already starting to bubble. “This metal is highly reactive to sunlight, and I will not have you getting chemical burns during our first ever lesson.” He kept the vial covered by his fist as he got up and walked around the desk and put this vial back in its protective cloth in the case. “I’ll give you one more try, the Morfis amanita _please_?”

“Here it is.” She reached in and grabbed the exact container with barely a glance at the case and held it out to Claude.

He shook his head and breathed out a laugh. “If you knew where it was, you should’ve handed it over. Now for the next step, I’ll grind these dried mushroom pieces down into a powder and add it in while the water is still boiling.” Claude had begun using the mortar and pestle while narrating, and Byleth added to her notes. Despite this not being a formal class, she had carefully written out the process and ingredients, and even drawn a small diagram of the setup. The water was boiling in a small glass, suspended on metal stakes around a small burner, affectionately dubbed the “tiny fire” in her notes. Claude finished his task and poured the powder into the water, causing the previously lavender colored liquid to turn a light pink.

“And we’re done!” Claude removed the glass with a pair of metal tongs and set it on a heat resistant cloth. “That will cause delayed blisters on someone’s skin if ingested. Consider lesson one a success.” Claude smirked and leaned back in his seat, “Any questions?” Byleth nodded.

“Where did you learn all this?”

“I did a lot of reading.”

“What will you do with it now?”

“Store it away for…safekeeping.”

“When do I get to make something?”

As Byleth asked this, her hands slowly reached toward the setup where the poison had been just a few moments ago. The tiny fire reflected in her eyes, her expression almost delirious and Claude felt his heart skip a beat. “W-woah there.” His voice cracked a bit as he quickly reached out and pulled the entire desk towards himself an inch. _Where did that reaction come from?_ “I don’t think the world is ready for ‘Chemist Byleth’ after one demonstration. You don’t know nearly enough about the ingredients, and I’m not confident enough to let you handle fire around me just yet. Can’t have you singeing off these good looks, now can I?” He regained his composure enough to throw her a wink and a (definitely not nervous) smile. But when their eyes connected again, her gaze was deep and attentive, and Claude felt a little self-conscious.

“You will have to keep teaching me then.” Byleth resolved, closing her notebook. She’d gotten used to his flirtatious nature after having spent quite a bit of time with him on the training grounds. She had thought poisons would be interesting when he’d mentioned it to her briefly, but she hadn’t expected to become so engrossed. She wanted to know the titles and effects of every ingredient in all those vials, she wanted to mix things together and watch the reactions, and she reeeally wanted to set something on fire, but she knew she had to keep that urged tucked away, lest Claude deem the ‘Ashen Demon’ too dangerous for his makeshift room workshop.

“Shall we make this our usual meeting time, Professor Claude?” She looked at him with one eyebrow raised slightly. She didn’t tease often, but she had become comfortable around the boy with sparkling emerald eyes and infinite curiosity. His endless well of questions and attempts to get a rise out of her during their supplemental sword lessons had warmed her up to him, and now that he’d followed though on his promise to teach her as well, she trusted him just a bit more.

“Hey,” Claude laughed and crossed his arms as he watched her get up from the desk, “You’re the Teach around here. I much prefer the role of student, but if you want to keep learning the arcane and mystic ways of poisons well, who am I to deny such a request from the beautiful woman of mystery herself?” He got up and bent into a deep bow while maintaining eye contact, so that he wouldn’t miss the subtle roll of her eyes and his smile deepened at her muttered “I’ll see you in the dining hall.” She made her usual graceful exit and Claude sat back down at his desk, running a hand through his hair. _Usual meeting time huh?_ His thoughts flitted back to Byleth’s eyes reflecting fire and he swallowed thickly. _I guess that means I’ll need to come up with a proper lesson plan. Preferably with less fire involved._

* * *

Hilda was skipping down the dormitory hallway, having just dumped her tasks for the day off on some unexpecting students. She was ready to go to her room and take a nap after all her hard work of delegating when one of the room doors up ahead opened and Byleth stepped out. “Oh, hey Byleth!” Hilda called out, realizing she hadn’t seen the quiet girl all day and that this must be where she’d been. Byleth turned with a start and realized she’d been standing outside the door to Claude’s room a bit longer than she had meant to. She started walking in the opposite direction, towards where Hilda had come from, walking past with a nod. “Hello Hilda. I’m heading to the library. See you in the dining hall later?”

“Of course!” Hilda’s face lit up at the thought. “They’re serving my… favorite…today…” she trailed off as Byleth passed right by her. _Is she- blushing?! _Hilda whipped her head around so fast she almost smacked herself in the face with her pigtails. “Okay, which room was it?” Hilda thought aloud. She walked back to the doorway she was 90% sure Byleth had just walked out of and -oh.

_Oh._

She was both surprised and not surprised to realize that whatever drew a reaction from Byleth’s face had absolutely been their House Leader’s doing. He’d been mildly obsessed with their mysterious new classmate since she’d joined the academy, and Hilda was certain Claude had developed a crush, despite his protests every time she asked him about his dear “Teach”.

“I cannot wait to tell Marianne about this.” Hilda grinned and continued down the hall to her room, deciding it could still wait until after her nap.

* * *

A few months later, Byleth sat in Claude’s room once again. Today marked the day of Byleth’s second ever experiment, after the first one had gone over relatively smoothly. It hadn’t come out right, but she hadn’t set anything on fire either, so Claude had deemed it a success.

Currently, he was leaning over her shoulder, watching her as she added some Albinean snake venom to a mixture of spices and other ingredients that should’ve had the mild sizzling effect she’d been asking to see, if his notes were correct. Not that he’d wanted to impress her or anything, but he was proud of her studiousness and self-control thus far and figured it was time for some excitement.

“Okay, now slowly add just a drop, and-“ But Byleth was a bit too eager, and instead of a careful measured drop, accidentally poured half the vial of venom.

“No wait-!” Claude reached out to grab her wrist and pull it back but it was already too late, the concoction sizzled quickly before bubbling up and popping spices and chemicals back at both their faces.

Byleth scrunched up her face, muffling her cry of surprise and now stinging pain on her face and Claude quickly grabbed two rags and threw them into a bucket of clean water he had started keeping in his room for emergencies like this after they had started their lessons and he realized his pupil was a bit of a wild card. He wiped off his face and hurriedly grabbed the other rag. “Hold still.” He pulled out the chair she sat on so he had access to her face and started with her eyes. “Don’t open your eyes until I say so, I don’t want you to get burned.”

“Okay.” She clenched her hands in her lap, anxiously waiting but trusting him to fix what she’d done. He carefully rubbed all around her eyes before sweeping out to the rest of her face. As he did, he realized he’d never seen her up this close before and with her eyes closed he could really see how long her eyelashes were. The worst of it gone, his touch got gentler as he admired her, and her face relaxed at the gentle sweeping of the rag on her cheeks and forehead. He’d realized she was pretty upon their first meeting; he had eyes of course. But now, inches away from her relaxed face, months spent together leading up to this moment of her putting her complete trust in him, he felt his mouth go dry. He looked down at her mouth and his movements slowed, and he thought back to when he had first seen her in action in Remire; how he had wanted her on his side to use her strength and battle experience to the Golden Deer’s advantage. _But now… _He hadn’t expected to enjoy her company as much as he did. He’d started to truly bond with all of his fellow Deer, but Byleth filled a hole in life he hadn’t even realized existed.

“Claude?”

“You can open your eyes.” The words rushed out of his mouth in embarrassment when he realized he’d been holding the rag to her cheek long enough to garner her attention. Her eyes opened and they both started at the proximity. Byleth looked from Claude to the mess on the table where the vial had knocked itself over and now sat lightly sizzling on the table. He followed her gaze, and when their eyes met again Byleth’s mouth twitched upwards at Claude’s growing smile before they both dissolved into a fit of giggles.

“You poured _half of the venom_\- “

“My hand slipped!”

“This coming from the girl who once cut off a tiny lock of Sylvain’s hair in a sparring match just to prove a point about sincerity.”

“Wait, you weren’t even there that da- “

“_All I’m saying_ is that your hand slipping is a bit of a flimsy excuse.” Claude shrugged, a goofy grin still on his face. “And now there’s acid fizzing on and likely t_hrough_ my table.”

Byleth’s eyes softened and she looked back to the table. “Yes, I am sorry about that part. Maybe I don’t have the touch for potions after all.” She sighed and he felt a pang in his heart. _No, this isn’t what he wanted when he had teased her earlier._

“Hey, third time’s the charm, yeah?” He reached out and ruffled her hair. She looked back up at him with hope in her eyes. “You’ll let me try again?”

“Do you think that was my first chemistry accident?” He asked with eyebrows raised. “How do you think I knew to keep a bucket of clean water handy?”

“… I honestly thought that was in case of fire.” She tilted her head and he chuckled.

“Well, that too, but trust me when I say I’ve had burns much worse than a couple spices are capable of causing.” He held out his hands for inspection, and Byleth noticed a few splotches of skin colored just a tad off from his normal skintone. “These are pretty recent so they haven’t fully healed yet, but I assure you they hurt a lot worse.”

She reached out and touched where the burns had left marks, and Claude froze.

“Hmm.” Effectively holding his hands, she inspected and prodded, wondering about his past experiments while he felt a blush creeping up his neck. The intimacy of the moment made him bold, and he thought about how she had looked lit up with laughter minutes ago contrasted with her thoughtful concentration now.

“Hey By?”

“Mm.” She hummed still looking at his hands in hers, having turned them over now and now inspecting the lines on his palms simply because he was doing nothing to stop her.

“Idon’twantthelessonstoendyet.” He’d said it so fast and in one exhaled breath that Byleth blinked and looked up at his face, trying to process his words.

When she puzzled out what he’d said and placed it against his look of vulnerability, she started to smile once more.

“Neither do I.”

Claude immediately relaxed. She knew. She knew what he meant. _He_ wasn’t even completely sure what he was trying to imply beyond _I want to spend more time with you_, but he was relieved to have refreshed his memory of why he had started to put a dangerous amount of trust in her, both on and off the battlefield.

A knock at the door made both of them jump. They locked eyes again in panic and had a quick, telepathic conversation.

_We aren’t supposed to be experimenting with poisons. What if it’s Professor Seteth?_

_I’ll take the hit if necessary, but you need to hide._

And with that he swiped everything off the table into an empty pail and placed stacks of books where the table had been ruined. They both moved to push the desk back towards the wall as silently and quickly as possible and Byleth hopped into a chest at the edge of Claude’s bed that he kept empty. This wasn’t their first secret lesson, and Seteth was pretty generous with detentions.

A second knock hit the door much harder and Claude tossed open a book on geography onto his bed before swinging the door open to reveal a gruff looking Jeralt on the other side. Claude gulped and schooled his face to his normal mask of casual friendliness. “Hello Captain. What can I do for you?”

Jeralt grunted and looked down at the boy who he’d noticed had gotten very comfortable around his daughter. “Have you seen Byleth? I asked a giggly pink-haired girl from your class and she said that normally at this time on Saturdays, she’s with _you_.” Jeralt’s mouth snarled just slightly on the last word, inciting a mild panic attack in Claude and his thoughts a jumble of _he’s going to find out he’s going to kill me I am in so much trouble Byleth can hear all of this from the chest-_

“Byleth?” Claude put on the most innocent expression he could possibly muster knowing he was about to lie through his teeth to his best friend’s father AND the most legendary knight of the Knights of Seiros. “It’s true we normally hang out right about now, but I haven’t seen her yet today. Maybe she had a last minute errand? Lots of other students ask for her help, whether it’s academic or friendly favors.”

Jeralt squinted hard at Claude’s face and debated whether or not to ask Claude’s intentions, but ultimately decided to trust in Byleth’s judge of character. For now. He was satisfied with the boy’s obvious intimidation, but also impressed at his composure despite it.

“Alright. I’ll keep asking around.” Jeralt walked away and Claude slowly closed the door and fell to the ground in relief at not having been caught. He looked over to the chest that hid Byleth and struggled to reign in his pulse with a hand over his heart. _The things I do for you._

In the chest, Byleth had heard the entire exchange, and muffled her giggles with a hand over her mouth. _Yes, _she thought_, I’d like our lessons to go on for quite a bit longer._

**Author's Note:**

> Thanks for reading! I don't have a plan for what order to continue this series in, but I have a few more ideas for our pre- and post-timeskip duo. The next scheme might introduce more characters if I feel brave enough to try to write them accurately, so...yeah. Hope you enjoyed.


End file.
